Ethanol is a viable, economical, and relatively clean fuel substitute or additive. It is easily obtained from the fermentation of grain or other substances containing sugars and starches. Although grain and other sugar-bearing substances are in abundance, the conversion of cellulosic material, such as found in municipal solid waste, to sugar followed by the fermentation of the sugar has been found useful for the purpose of obtaining ethanol. The use of such waste cellulose has been particularly attractive in the face of higher grain costs and concerns about waste disposal.
Cellulosic material generally includes waste paper, agricultural chafe, municipal solid waste residual fluff, and wood products. These substances are converted to sugar via hydrolysis. Heretofore in the art, cellulosic material has been hydrolyzed by first reducing the material to a pulp and reacting that pulp with sulfuric acid. Upon the introduction of heat, hydrolysis begins and the cellulosic material is converted to sugar. The reaction is quenched by rapid cooling of the mixture, followed by acid neutralization. Rapid quenching is necessary because the hydrolysis reaction is virtually instantaneous, and over exposure to heat and acidic conditions will result in the decomposition of the sugar product thereby reducing yield.
This process, however, is thermally inefficient because the heat introduced to the system is lost through the rapid cooling of the system. Furthermore, inefficiencies resulting from the use of thick pulp solutions of cellulosic material, which conventionally contains approximately 20% suspended solids, have been recognized. Specifically, these solutions require screw augers to accomplish the required mixing of acid and heat. Thus, when a reaction vessel is 1,000 cubic feet, which is the minimum for commercial quantities, the time to achieve uniform mixing can be as long as twenty minutes. In addition to the inefficiencies associated with powering the auger, this process will result in poor sugar yields as the time required to uniformly mix the pulp is typically too long, resulting in the decomposition of the resulting sugar.
Although the problems associated with the use of thick pulps can be overcome by simple dilution with water, the added energy required to handle such liquid results in further inefficiencies. Indeed, the total energy required to produce ethanol via such a process is greater than the heat of combustion of the resulting ethanol.
Thus, a need exists to convert cellulosic material to sugar for the purpose of obtaining ethanol in an efficient manner. Specifically, to create an economical fuel substitute or additive, the thermal and chemical inefficiencies associated with the processes of hydrolysis described hereinabove must be overcome.
Numerous methods and reactions for carrying out hydrolysis are known in the art. For example, Titmas in U.S. Pat. Nos. 3,853,759 and 4,792,408 discloses a continuously flowing hydraulic column wherein materials suspended in water are heated and gravity pressurized to effect hydrolysis. The heated material is forced upward by column pressure and thereby cooled and depressurized. Although this process could handle large quantities of cellulosic material, poor net sugar yield would be obtained because there is no means to control or manipulate the length of the hydrolysis reaction, nor is there any means to abruptly and specifically control the quenching of the reaction. To achieve satisfactory sugar yields, the hydrolysis reaction must be stopped part way through the normal coarse of chemical events. This has not been accomplished heretofore in the art.
Also, Pavilon, U.S. Pat. No. 5,135,861, discloses a method of producing ethanol from an aqueous slurry of biomass. The carbon dioxide resulting from fermentation is captured and used to catalyze the hydrolysis of the biomass. Pavilon, however, fails to efficiently utilize the heat needed for hydrolysis to catalyze further hydrolysis reactions, and thus the energy needed to convert the biomass to ethanol is greater than the resulting heat of combustion of the ethanol.
Thus, there remains a need for a method and apparatus for the conversion of cellulosic material to ethanol. Specifically, there remains a need for a method an apparatus for the efficient hydrolysis of cellulosic materials which includes improving sugar yield and capturing the heat needed for hydrolysis to further additional hydrolysis which will in turn result in a process that is both cost effective and thermodynamically efficient.